You must supply this method. It should return a tuple (x,
y) giving the current position of the scrollbars (between
0 and 32767). You can return None for either to
indicate the whole document is visible in that direction.

Auxiliary method to help you calculate values to return from
getscrollbarvalues(). You pass document minimum and maximum value
and topmost (leftmost) and bottommost (rightmost) visible values and
it returns the correct number or None.